Solid state image sensing element improved in sensitivity and production cost, process of fabrication thereof and solid state image sensing device using the same

ABSTRACT

A solid state image sensing element has a miniature lens buried in a transparent interlayer insulating layer over a photo diode formed in a semi-conductor substrate, and the miniature lens occupies an area wider than an area occupied by the photo diode so that the solid state image sensing element is sensitive without sacrifice of production cost.

This is a divisional of application Ser. No. 09/056,858 filed Apr. 8,1998, now U.S. Pat. No. 6,104,021, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a solid state image sensing device and, moreparticularly, to a solid state image sensing element, a process offabrication thereof and a solid state image sensing device equipped withthe solid state image sensing element.

DESCRIPTION OF THE RELATED ART

A CCD (Charge Coupled Device) type image sensing device is a typicalexample of the solid state image sensing device, and is describedhereinbelow. However, the following description is applicable to anotherkind of solid state image sensing device such as a MOS(Metal-Oxide-Semiconductor) type solid state image sensing device.

FIG. 1 illustrates the first prior art slid state image sensing device.A p-type well 1 is formed in a surface portion of an n-type siliconsubstrate 2, and an n-type impurity region 3 is nested in the p-typewell 1. A heavily-doped p-type impurity region 4 is formed over then-type impurity region 3, and the heavily-doped p-type impurity region 4and the n-type impurity region 3 form a p-n junction serving as a photodiode.

An n-type charge transfer region 5 is further formed in the p-type well1, and is spaced from the photo diode, i.e., the n-type impurity region3 and the heavily-doped p-type impurity region 4. Though not shown inFIG. 1, photo diodes are arranged along the n-type charge transferregion 5, and the photo diodes and the n-type charge transfer region 5form in combination an image sensing line. A heavily doped p-typeimpurity region 6 is formed in such a manner as to of the image sensingline, and electrically isolates the photo diodes and the n-type chargetransfer region 5 from adjacent image sensing lines. Thus, a largenumber of photo diodes are arrayed in the p-type well 1. However,description is focused on only one of the photo diodes and the n-typecharge transfer region 5.

A read-out transistor 7 is associated with the photo diode and then-type charge transfer region 5. In detail, a surface portion of thep-type well 1 between the photo diode and the n-type charge transferregion 5 provides a channel region 7 a, and the channel region 7 a iscovered with a gate oxide layer 7 b. A charge transfer electrode 7 c isformed on the gate oxide layer 7 b, and is covered with a silicon oxidelayer 8. The silicon oxide layer 8 is over-lain by a photo shield layer9, and an opening 9 a is formed in the photo shield layer 9 over thephoto diode. For this reason, image-carrying light is incident onto thephoto diode through the opening 9 a, and the n-type charge transferregion 5 is prevented from the light.

The photo shield layer 9 is covered with a transparent insulating layer10, and the opening 9 a is filled with the transparent material. A thickphoto resist layer 11 is laminated on the transparent insulating layer10, and provides a flat upper surface 11 a. An on-chip lens 12 is formedon the flat upper surface 11 a, and is located over the photo diode soas to focus the image carrying light on the photo diode. The thick photoresist layer 11 is made from photo resist solution through a baking. Theon-chip lens 12 is also made from a piece of photo resist. A photoresist layer is patterned into pieces of photo resist throughlithographic techniques, and the piece of photo resist thermally curedat 150 degrees to 200 degrees in centigrade. Then, the piece of photoresist is shaped into a semi-spherical configuration as shown.

The second prior art solid state image sensing device is disclosed inJapanese Patent Publication of Unexamined Application (JPA) No. 2-65171,and FIG. 2 illustrates the second prior art solid state image sensingdevice. A p-type well 21 is formed in a surface portion of an n-typesilicon substrate 22, and an n-type impurity region 23 is nested in thep-type well 21. A heavily-doped p-type impurity region 24 is formed overthe n-type impurity region 23, and the heavily-doped p-type impurityregion 24 and the n type impurity region 23 form a p-n junction servingas a photo diode.

An n-type charge transfer region 25 is further formed in the p-type well21, and is spaced from the photo diode. The photo diode and the n-typecharge transfer region 25 form an image sensing line together with otherphoto diodes. A heavily doped p-type impurity region 26 is formed insuch a manner as to surround the image sensing line, and electricallyisolates the photo diodes and the n-type charge transfer region 25 fromadjacent image sensing lines.

A read-out transistor 27 is associated with the photo diode and then-type charge transfer region 25, and comprises a channel region 27 a, agate oxide layer 27 b over the channel region 27 a and a charge transferelectrode 27 c formed on the gate oxide layer 27 b. The charge transferelectrode 279 is covered with a silicon oxide layer 28, and the siliconoxide layer 28 is overlain by a photo shield layer 29. An opening 29 ais formed in the photo shield layer 29 over the photo diode, and allowsimage-carrying light to be incident onto the photo diode through theopening 29 a. The photo shield layer 29 prevents the n-type chargetransfer region 25 from the incident light. The photo shield layer 29 istopographically covered with a transparent insulating layer 30, and thetransparent insulating layer 30 forms a deep recess 30 a. The deeprecess 30 a is located over the photo diode. The deep recess 30 a ispartially filled with silica glass, and the piece of silica glass 31forms a curved upper surface 32. The curved upper surface 32 forms ashallow recess nested in the deep recess 30 a. The shallow recess isfilled with silicon nitride, and the silicon nitride has a refractiveindex larger than the silica glass. For this reason, the piece ofsilicon nitride 33 serves as a lens. The upper surface of the lens 33 isplanarized as shown.

The on-chip lens 12 occupies the wide area over the photo diode 3/4 andthe n-type charge transfer region 5, and gathers the incident lightfallen thereonto. For this reason, the photo diode 3/4 is sensitive tothe variation of the incident light. However, the first prior art solidstate image sensing device encounters a problem inhigh price. Asdescribed hereinbefore, the on-ship lens 12 is formed of photo resistsolidified through the baking, and, accordingly, is brittle. The brittleon-chip lens is liable to be broken during the fabrication of the firstprior art solid state image sensing device, and decreases the productionyield. This makes the price of the first prior art solid state imagesensing device high.

Another reason for the high price is serious influences of dust. Theon-chip lenses 12 project from the flat upper surface 11 a of the photoresists layer 11, and form valleys therebetween. If a dust particlefalls into the valley, the dust particle is hardly eliminated from thevalley, and makes the product defective. For this reason, the firstprior art solid state image sensing device requires extremely highcleanliness, and such an extremely high clean ambience increases theproduction cost of the first prior art solid state image sensing device.

Yet another reason for the high price is a complicated packagingstructure. The on-chip lens 12 has the exposed curved surface. If theexposed curved surface is held in contact with transparent layer whichhas a large refractive index, the on-ship lens 12 loses the convergentfunction. For this reason, the on-chip lens 12 is required to be exposedto the air, or is covered with an extremely low refractive indexmaterial layer. The manufacturer takes these requirements into account,and designs the package for the first prior art solid state imagesensing device. The package is complicated, and increases the productioncost.

The second prior art solid state image sensing device is less costly,because the curved surface of the lens 33 is embedded into the piece ofsilica glass 31. However, the second prior art solid state image sensingdevice suffers from a low sensitivity. The low sensitivity is derivedfrom the small lens 33. The shallow recess defines the lens 33, and theshallow recess is defined in the deep recess 30 a. The deep recess 30 ais defined by the transparent insulating layer topographically extendingon the photo shield layer 29 around the opening 29 a, and only the photodiode 23/24 is exposed to the opening 29 a. The n-type charge transferregion 25 is never exposed to the openings 29 a. For this reason, thelens 33 merely occupies an area over the photo diode 23/24, and can notgather the incident light fallen over the n-type charge transfer region25. Thus, there is a trade-off between the first prior art solid stateimage sensing device and the second prior art image sensing device.

SUMMARY OF THE INVENTION

It is therefore an important object of the present invention to providea solid state image sensing element, which is low in production cost andhigh in sensitivity.

It is another important object of the present invention to provide aprocess of fabricating the solid state image sensing element.

It is yet another important object of the present invention to provide asolid state image sensing device in which the solid state image sensingelement serves as an essential component.

In accordance with one aspect of the present invention, there isprovided a solid state image sensing element fabricated on a substratecomprising a photo-electric converting element occupying a first area ofthe substrate and converting incident light to photo carrier, a firsttransparent layer covering the photo-electric converting element, formedof a first transparent material and having a first recess occupying asecond area wider than the first area and a second transparent layerprovided in the first recess and formed of a second transparent materiallarger in refractive index than the first transparent material so as toserve as a lens.

In accordance with another aspect of the present invention, there isprovided a process for fabricating a solid state image sensing element,comprising the steps of preparing a substrate, forming a photo-electricconverting element in a first area of the substrate, covering thephoto-electric converting element with a first transparent layer formedof a first transparent material, forming a mask layer on the firsttransparent layer having an opening over a central sub-area of the firstarea, isotropically etching the first transparent so as to form a firstrecess and filling the first recess with a second transparent materiallarger in refractive index than the first transparent material so as toform a second transparent layer serving as a lens.

In accordance with yet another aspect of the present invention, there isprovided a solid state image sensing device comprising a substrate, anarray of solid state image sensing elements fabricated on the substrateand including a plurality of photo-electric converting element eachoccupying a first area of the substrate and converting incident light tophoto carrier, a first transparent layer covering the plurality ofphoto-electric converting elements, formed of a first transparentmaterial and having first recesses each occupying a second area widerthan the first area over one of the plurality of photo-electricconverting elements and a plurality of second transparent layersrespectively provided in the first recesses and formed of a secondtransparent material larger in refractive index than the firsttransparent material so as to serve as lenses, respectively, and apackage accommodating the substrate and having a transparent portionwith an inner surface held in contact with the array of solid stateimage sensing elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the solid state image sensing element,the fabrication process and the solid state image sensing device will bemore clearly understood from the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a cross sectional view showing the structure of the firstprior art solid state image sensing device;

FIG. 2 is a cross sectional view showing the structure of the secondprior art solid state image sensing device;

FIG. 3 is a plane view showing a solid state image sensing elementfabricated on a semiconductor substrate according to the presentinvention;

FIG. 4 is a cross sectional view taken along line A—A of FIG. 3 andshowing the structure of the solid state image sensing element;

FIGS. 5A to 5C are cross sectional views showing essential steps of aprocess for fabricating the solid state image sensing element accordingto the present invention;

FIG. 6 is a cross sectional view showing the structure of another solidstate image sensing element according to the present invention;

FIG. 7 is a cross sectional view showing the structure of yet anothersolid state image sensing element according to the present invention;

FIG. 8 is a cross sectional view showing the structure of still anothersolid state image sensing element according to the present invention;

FIG. 9 is a cross sectional view showing a semi-elliptical recess formedunder a photo resist layer;

FIG. 10 is a graph showing relation between an etching rate and a depthfrom the boundary between a transparent layer and the photo resistlayer;

FIG. 11 is a graph showing relation between an etching rate and a depthfrom the boundary between another transparent layer and a photo resistlayer;

FIG. 12 is a cross sectional view showing a semi-spherical recess formedin the transparent layer;

FIG. 13 is a plane view showing the layout a solid state image sensingdevice according to the present invention;

FIG. 14 is a cross sectional view taken along line B—B of FIG. 13 andshowing the structure of the solid state image sensing device;

FIG. 15 is a plane view showing the layout another solid state imagesensing device according to the present invention; and

FIG. 16 is a cross sectional view taken along line C—C of FIG. 15 andshowing the structure of the solid state image sensing device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

Referring to FIGS. 3 and 4 of the drawings, a solid state image sensingelement 30 embodying the present invention largely comprises a photodiode 31 and a buried miniature lens 32. The solid state image sensingelement 30 form a solid state image sensing device together with othersolid state image sensing elements, shift registers 33 and aphoto-shield structure 34. The other solid state image sensing elementsare similar in structure to the solid state image sensing element 30,and make a plurality of image sensing lines. Each image sensing lineincludes a plurality of solid state image sensing elements. The shiftregisters 33 are respectively associated with the image sensing lines,and FIG. 3 illustrates the solid state image sensing element 30incorporated in one of the image sensing lines and the associated shiftregister 33. The solid state image sensing element 30 and the shiftregister 34 are hereinbelow described in detail.

A p-type well 35 is formed in a surface portion of an n-type siliconsubstrate 36, and the image sensing line and the associated shiftregister 33 are formed in the p-type well 35. A heavily doped p-typeimpurity region 37 isolates the image sensing line and the associatedshift register 33 from the adjacent image sensing line/shift register.

An n-type impurity region 31 a and a heavily-doped p-type region 31 bform a p-n junction 31 c serving as the photo diode, and convertsincident light to photo carrier. The amount of photo carrier isproportional to the intensity of the incident light. The photo carrieris accumulated in the n-type impurity region 31 a.

The shift register 33 includes an n-type charge transfer region 33 aformed in the p-type well 35 and charge transfer electrodes 33 bpatterned on a thin insulating layer 33 c covering the p-type well 35.The n-type charge transfer region 33 a extends along the image sensingline, and is spaced from the n-type impurity region 31 a by a channelregion 33 d. Predetermined charge transfer electrodes 33 b projects fromthe area over the n-type charge transfer region 33 a to the area overthe channel region 33 d as shown in FIG. 4, and the predetermined chargetransfer electrodes 33 b are electrically connected to a read-out signalline (not shown). When a read-out pulse is applied to the predeterminedcharge transfer electrode 33 b, a conductive channel is formed betweenthe n-type impurity region 31 a and the n-type charge transfer region 33a, and the photo carrier is read out from the photo diode to the shiftregister 33.

The photo shield structure 34 includes a non-transparent photo shieldlayer 34 a sandwiched between a silicon oxide layer 34 b and atransparent insulating layer 34 c. The photo shield layer has an opening34 d, and a part of the thin insulating layer 33 c over theheavily-doped p-type region 31 b is exposed to the opening 34 d. Thetransparent insulting layer 34 c topographically extends over the photoshield layer 34 a, and the area of the thin insulating layer 33 c iscovered with the transparent insulating layer 34 c. The photo diode 31,the shift register 33 and the photo shield structure 34 are similar tothose of the prior art already described.

The photo shield structure 34 is covered with a thick transparent layer38, and a generally semi-spherical recess 38 a is formed in the surfaceportion of the thick transparent layer 38. The generally semi-sphericalrecess 38 a occupies an area over the photo diode 31, the channel region33 d and a part of the n-type charge transfer region 33 a as will beseen in FIG. 3. The upper surface 38 b between the generallysemi-spherical recesses 38 a is flat. The buried miniature lens 32 isshaped into a generally semi-spherical configuration corresponding tothe generally semi-spherical recess 38 a , and is snugly received in thegenerally semi-spherical recess 38 a . Thus, the buried miniature lens32 is much wider than the opening 34 d. The buried miniature lens 32 isformed of transparent material larger in refractive index than thetransparent material forming the thick transparent layer 38.

The buried miniature lens 32 has a flat upper surface 32 a, and the flatupper surface 32 a is substantially coplanar with the upper surface 38 bof the thick transparent layer 38. The upper surfaces 32 a and 38 b arecovered with a transparent protective layer 39 of silicon nitride.

As will be understood from the foregoing description, the buriedminiature lens 32 directs the incident light falling over the channelregion 33 d and the part of the n-type charge transfer region 33 a tothe photo diode 31, and surely improves the sensitivity. The buriedminiature lens 32 provides the flat upper surface 32 a coplanar with theupper surface 38 b of the thick transparent layer 38, and any valley isnot formed between the buried miniature lens 32 and the thicktransparent layer 38. For this reason, a dust particle is easilyremovable, and does not seriously damages the solid state image sensingdevice. This means that the solid state image sensing element is not sosensitive to the environment. The transparent protective layer preventsthe buried miniature lens from undesirable force, and the buriedminiature lens is not easily broken. Thus, the solid state image sensingelement allows the manufacturer to reduce the production cost of thesolid state image sensing device.

The solid state image sensing element 30 is fabricated through a processillustrated in FIGS. 5A to SC. The process starts with preparation ofthe n-type silicon substrate 36. Photo resist solution is spread overthe major surface of the n-type silicon substrate 36, and is baked so asto form a photo resist layer (not shown). A pattern image is transferredfrom a photo mask (not shown) to the photo resist layer, and forms alatent image in the photo resist layer. The latent image is developed soas to pattern the photo resist layer into a photo resistion-implantation mask (not shown). Thus, the photo resistion-implantation mask is formed from the photo resist layer through thelithographic techniques.

A predetermined surface area of the n-type silicon substrate 36 isexposed to an opening of the photo resist ion-implantation mask, andp-type dopant impurity is ion implanted into the exposed surface portionof the n-type silicon substrate 36. The ion-implanted p-type dopantimpurity forms the p-type well 35. The photo resist ion-implantationmask is stripped off.

A photo resist ion-implantation mask (not shown) is formed on the n-typesilicon substrate 36 by using the lithographic techniques, and a surfaceportion of the p-type well 35 is exposed to an opening of the photoresist ion-implantation mask. The surface portion is assigned to theheavily doped p-type impurity region 37. P-type dopant impurity is ionimplanted into the exposed surface portion, and forms the heavily dopedp-type impurity region 37 for the electrical isolation.

Similarly, the n-type impurity region 31 a, the n-type charge transferregion 33 a and the heavily-doped p-type impurity region 31 b are formedby using the lithographic techniques and the ion-implantation. Thesurface portion between the n-type impurity region 31 a and the n-typecharge transfer region 33 a provides a channel region 33 d for readingout the photo carrier. The resultant semiconductor structure of thisstage is shown in FIG. 5A.

Subsequently, the thin insulating layer 33 c is formed over the uppersurface of the resultant semiconductor structure. The thin insulatinglayer 33 c is formed from a single silicon oxide film or a combinationof silicon oxide film and a silicon nitride film. Phosphorous-dopedpolysilicon is deposited over the entire surface of the thin insulatinglayer 33 c, and a photo resist etching mask (not shown) is formed on thephosphorous-doped polysilicon layer by using the lithographictechniques. The phosphorous-doped polysilicon layer is selectivelyetched away, and is formed into the predetermined charge transferelectrode 33 b. The predetermined charge transfer electrode 33 b isthermally oxidized, and is covered with silicon oxide. Thedoped-polysilicon is deposited over the entire surface of the resultantsemiconductor structure, and a photo resist etching mask (not shown) isformed on the doped-polysilicon layer by using the lithographictechniques. The doped polysilicon layer is selectively etched away, andis formed into the other charge transfer electrodes (not shown).

Silicon oxide is grown over the other charge transfer electrodes, andforms the silicon oxide layer 34 b together with the silicon oxidethermally grown on the predetermined charge transfer electrode 33 b.Non-transparent material is deposited to 200 nanometers to 500nanometers thick over the resultant semi-conductor structure, and aphoto resist etching mask (not shown) is formed on the non-transparentmaterial layer by using the lithographic techniques. The non-transparentmaterial layer may be an aluminum layer, a tungsten layer or a compositelayer thereof Using the photo resist etching mask, the non-transparentmaterial layer is selectively etched away, and is formed into thephoto-shield layer 34 a. The non-transparent material layer is removedfrom the predetermined area of the thin insulating layer 33 c over thephoto diode 31, and, accordingly, the photo shield layer 34 a has theopening 34 d over the photo diode 31. Subsequently, transparentinsulating material is deposited over the entire surface of theresultant semiconductor structure, and forms the transparent insulatinglayer 34 c as shown in FIG. 5B. The process sequence is similar to theprior art process for fabricating the prior art solid state imagesensing device until the semiconductor structure shown in FIG. 5B.

Subsequently, silicon oxide is deposited to 2 microns to 5 microns overthe entire surface of the resultant semiconductor structure, and forms ssilicon oxide layer. The thick transparent layer 38 is formed from thesilicon oxide layer. The focal length of the buried miniature lens 32 istaken into account for determining the thickness of the silicon oxidelayer. Phosphorous or boron may be introduced into the silicon oxide.The phosphorous or the boron prevents the thick transparent layer 38from stress, and a crack is less liable to take place in the thicktransparent layer 38. Otherwise, a plurality of transparent layersdifferent in thermal expansion coefficient may be laminated so as torelief the thick transparent layer 38 from the internal stress.Moreover, the silicon oxide layer may be subjected to a chemicalmechanical polishing so as to enhance the flatness of the upper layer 38b.

A photo resist etching mask 40 is provided on the thick transparentlayer 38, and has an opening 40 a over the photo diode 31. Using thephoto resist etching mask 40, the thick transparent layer 38 isisotropically etched. The etchant may be dilute hydrofluoric acid. Thegenerally semi-spherical recess 38 a is formed in the thick transparentlayer 38 through the isotropic etching as shown in figure 5C. It ispossible to continue the isotropic etching just before separation of thephoto resist etching mask 40 from the thick transparent layer 38. In theactual process, the manufacturer takes the optical characteristics ofthe buried miniature lens 32 into account, and optimizes the etchingtime. A suitable mask layer such as a silicon nitride layer may beformed between the photo resist etching mask and the thick transparentlayer 38. In this instance, the photo resist etching mask 40 is used forpatterning the mask layer, and the generally semi-spherical recess 38 ais formed through the isotropic etching using the mask layer.

Subsequently, transparent material is deposited over the thicktransparent layer 38. The transparent material fills the generallysemi-spherical recess 38 a, and swells into a transparent layer. Thetransparent layer is chemically mechanically polished until the uppersurface 38 b is exposed, and the buried miniature lens 32 is left in thegenerally semi-spherical recess 38 a. The transparent material for theburied miniature lens 32 is larger in refractive index than the materialforming the thick transparent layer 38. In this instance, the thicktransparent layer 38 is formed of silicon oxide, and silicon nitride isdeposited over the thick transparent layer 38. The refractive index ofthe silicon oxide is of the order of 1.6, and the silicon nitride hasthe refractive index of 2.0.

The radius of curvature of the generally semi-spherical recess 38 a andthe ratio of refractive index between the thick transparent layer 38 andthe buried miniature lens 32 determine the focal length. Themanufacturer appropriately selects the transparent material for thethick transparent layer 38, the transparent material for the buriedminiature lens 32, the etching time and the thickness of the thicktransparent layer 38 so as to optimize the focal length of the buriedminiature lens 32.

Finally, the protective layer 39 is formed over the thick transparentlayer 38 and the buried miniature lens 32. The protective layer 39should be hard. In this instance, the protective layer 39 is formed ofsilicon nitride. Then, the solid state image sensing element shown inFIGS. 3 and 4 is fabricated on the n-type silicon substrate 36.

Second Embodiment

Turning to FIG. 6 of the drawings, another solid state image sensingelement 50 embodying the present invention is fabricated on an n-typesilicon substrate 51 together with a shift register 52. The shiftregister 52 is similar to that of the first embodiment, and the solidstate image sensing element 50 is only different from the solid stateimage sensing element 30 in the structure of a buried miniature lens 53.For this reason, the component elements of the shift register 52 and theother component elements of the solid state image sensing element 50 arelabeled with the same references designating corresponding elements ofthe shift register 33 and the corresponding elements of the solid stateimage sensing elements 30, and description is focused on the structureof the buried miniature lens 53 for the sake of simplicity.

The thick transparent layer 38 has a generally semi-spherical recess 38c that is partially filled with silicate glass, which is sometimesexpressed as “SOG (Spin-On-Glass)”, and the silicate glass forms a firsttransparent layer 53 a. The first transparent layer 53 a has a lowercurved surface 53 b and an upper curved surface 53 c, and the lowercurved surface 53 b is merged with the upper curved surface 53 c alongthe periphery of the first transparent layer 53 a. Therefore, the firsttransparent layer 53 a is thickest at the center thereof, and thethickness is decreased from the center toward the periphery. The lowersurface 53 b has the radius of curvature equal to that of the generallysemi-spherical recess 38 c, and the upper surface 53 c has the radius ofcurvature greater than that of the lower surface 53 b. Although thefirst transparent layer 53 a does not participate in the convergence oflight because of fact that the refractive index of the silicate glass isapproximately equal to that of the silicon oxide, the first transparentlayer 53 a modifies the radius of curvature of a second transparentlayer 53 d serving as a convex lens.

As described hereinbefore, the first transparent layer 53 a has thecurved upper surface 53 c, which defines a shallow recess 53 d nested inthe generally semi-spherical recess 38 c. The shallow recess 53 d isalmost a generally semi-ellipsoid configuration. The shallow recess 53 dis filled with transparent material such as, for example, siliconnitride greater in refractive index than the silica glass. The siliconnitride forms the second transparent layer 53 d, and the secondtransparent 53 d serves as a convex lens. The convex lens of the secondtransparent layer 53 d has a flat upper surface 53 e substantiallycoplanar with the upper surface 38 b of the thick transparent layer 38,and the focal length is longer than that of the buried miniature lens32. Thus, the radius of curvature of the upper surface 53 c ischangeable as will be described hereinafter, and the manufacturer canadjusts the focal length by changing not only the thickness of the thicktransparent layer 38 but also the first transparent layer 53 a.

The first transparent layer 53 a is formed as follows. First, silicaglass solution is prepared. The silica glass solution is spread over thethick transparent layer 38, and fills the generally semi-sphericalrecess 38 c. The silica glass solution is baked. Then, the silica glasslayer is shrunk, and forms the curved upper surface 53 c. The shrinkageratio is variable together with the water content of the silica glasssolution. If the shrinkage ratio is 1:2, the generally semi-ellipsoidrecess has the ratio of the minor axis to the major axis=1:2, and thefocal length is twice increased rather than that of the generallysemi-spherical lens. The first transparent layer 53 a modifies the focallength of the convex lens, and serves as a focal length modifier.

When using the buried miniature lens 53, the designer easily optimizesthe optical characteristics for the photo diode 31. Because there arevarious design factors independently changeable, i.e., the ratio ofrefractive index between the transparent material for the thick layer 38and the transparent material for the convex lens 53 d, the radius ofcurvature of the surface defining the generally semi-spherical recess 38c, the thickness of the transparent layer 38, the ratio of refractiveindex between the transparent material for the first transparent layer53 a and the transparent material for the convex lens 53 d, theconfiguration of the curved upper surface 53 c and the thickness of eachtransparent layer 38/53 a/53 d affect the optical characteristics of thesolid state image sensing element 50, and the designer independentlychanges these factors.

Third Embodiment

FIG. 7 illustrates yet another solid state image sensing element 60embodying the present invention. The solid state image sensing element60 is fabricated on an n-type silicon substrate 61 together with a shiftregister 62. The shift register 62 is similar to that of the firstembodiment, and the solid state image sensing element 60 is onlydifferent from the solid state image sensing element 30 in the structureof a buried miniature lens 63. For this reason, the component elementsof the shift register 62 and the other component elements of the solidstate image sensing element 60 are labeled with the same referencesdesignating corresponding elements of the shift register 33 and thecorresponding elements of the solid state image sensing elements 30, anddescription is focused on the structure of the buried miniature lens 63for the sake of simplicity.

The thick transparent layer 38 has a generally semi-spherical recess 38d, and the buried miniature lens 63 is provided in the generallysemi-spherical recess 38 d. The buried miniature lens 63 is implementedby a transparent layer 63 a. A lower surface 63 b of the transparentlayer 63 a follows the curved surface defining the generallysemi-spherical recess 38 c, and an upper surface 63 c is also curved soas to define a shallow recess. The upper surface 63 c is larger inradius of curvature than the lower surface 63 b, and the transparentlayer 63 a is increased in thickness from the periphery to the centerthereof. For this reason, the transparent layer 63 a serves as a convexlens.

The solid state image sensing element 63 is fabricated as follows. Aprocess for fabricating the photo-electric element 63 is similar to thatof the first embodiment until the step of forming the generallysemi-spherical recess 38 d. Solution of mixture of TiO₂ and silica glassor SrTiO₃ and silica glass is, by way of example, prepared, and isspread over the entire surface of the thick transparent layer. The layerof the solution is baked so as to form the transparent layer 63 a. Whenthe mixture of TiO₂ and silica glass is used, the transparent layer 63 ahas the refractive index ranging between 2.3 and 2.5, which is greaterthan the refractive index of the silicon oxide forming the thicktransparent layer 38. Finally, the resultant semi-conductor structure iscovered with the protective layer 39, which topographically extends overthe transparent layer 63 a.

In this instance, the shrinkage ratio is variable as similar to thesilica glass used in the second embodiment, and the focal length isappropriately regulable. The solution of mixture does not form a flatupper surface. However, the transparent layer 63 a is harder than thebaked photo resist layer, and is not easily damaged. Moreover, theprotective layer 39 prevents the convex lens 63 a from scratches due toa dust particle. For this reason, even if a dust particle is left in theshallow recess, the dust-particle is easily eliminated from the shallowrecess.

The solid state image sensing element 60 forms a part of a solid stateimage sensing device, and the solid state image sensing device may besealed in a plastic package. Even though the plastic package does notform a space over the protective layer 39, the upper surface 63 c and,accordingly, the protective layer 39 have a large radius of curvature,and the plastic package does not widely change the focal plane. Theplastic package affects the radius of curvature. However, the variationis a little and predictable. For this reason, the manufacturer takes thevariation into account so as to determine the design factors of theburied miniature lens 63.

Fourth Embodiment

Turning to FIG. 8 of the drawings, still another solid state imagesensing element 70 is fabricated on an n-type silicon substrate 71together with a shift register 72. The shift register 72 is similar tothat of the first embodiment, and the solid state image sensing element70 is only different from the solid state image sensing element 30 inthe structure of a buried miniature lens 73. For this reason, thecomponent elements of the shift register 72 and the other componentelements of the solid state image sensing element 70 are labeled withthe same references designating corresponding elements of the shiftregister 33 and the corresponding elements of the solid state imagesensing elements 30, and description is focused on the structure of theburied miniature lens 73 for the sake of simplicity.

A thick transparent layer 74 is formed over the photo diode 31 and theshift register 72, and a generally semi-ellipsoid recess 74 a is formedin the thick transparent layer 74 over the photo diode 31. A transparentlayer 73 a fills the generally semi-ellipsoid recess 74 a, and serves asa convex lens.

The solid state image sensing element 70 is fabricated as follows. Aprocess for fabricating the solid state image sensing element 70 issimilar to the process shown in FIGS. 5A to 5C until the step of formingthe transparent insulating layer 34 c. Undoped silicon oxide isdeposited over the entire surface of the resultant semiconductorstructure, and phosphorous is ion implanted into a surface portion ofthe undoped silicon oxide layer. Otherwise, phosphorous-containingadditive gas such as PH₃ is mixed with the material gas such as SiH₄,and the amount of the phosphorous-containing additive gas is controlledin such a manner as to increase the concentration toward the uppersurface of the silicon oxide layer. As a result, a thickphosphorous-doped silicon oxide layer 74 b is formed on the transparentinsulating layer 34 c.

Subsequently, a photo resist etching mask 75 is formed on the uppersurface of the phosphorous-doped silicon oxide layer 74 b (see FIG. 9),and has an opening 75 a over the photo diode 31. Using dilutehydrofluoric acid, the phosphorous-doped silicon oxide layer 74 b isisotropically etched away. The etching rate is proportional to thephosphorous concentration, and the etching rate is gradually decreasedfrom the upper surface of the phosphorous-doped silicon oxide layer 74 btoward the inside thereof as shown in FIG. 10. For this reason, theisotropic etching widely proceeds in the lateral direction, and R1becomes greater than R2. As a result, the generally semi-ellipsoidrecess 74 a is formed in the phosphorous-doped silicon oxide layer 74 b.If a transparent layer has a constant dopant concentration as shown inFIG. 11, the etching rate is constant over the thickness of thetransparent layer, and the etchant forms a semi-spherical recess or agenerally semi-spherical recess 81 in the transparent layer 82 as shownin FIG. 12. When the dopant profile is changed, the generally ellipticalrecess varies the configuration. Thus, the manufacturer can optimize theoptical properties of the buried miniature lens 73 a by controlling theconditions of the ion-implantation or the conditions of the chemicalvapor deposition.

Subsequently, transparent material is formed over the transparent layer74. The transparent material fills the generally ellipsoid recess 74 a,and the transparent material layer is planarized. The chemicallymechanically polishing may be used. The transparent material is largerin refractive index than the phosphorous-doped silicon oxide, and thepiece of transparent material in the recess 74 a serves as a convexlens. Finally, the protective layer 39 is formed.

Solid State Image Sensing Device

Each of the solid state image sensing elements 30, 50, 60 and 70 isavailable for a solid state image sensing device. The solid state imagesensing device is broken down into a semi-conductor chip 90 and aplastic package 91 as shown in FIG. 13. The semi-conductor chip 90 isdivided into a central area 90 a and a peripheral area 90 b. The centralarea 90 a is assigned to an array of solid state image sensing elementsand vertical shift registers, and the peripheral area 90 b is assignedto a horizontal shift register and an amplifier. A kind of solid stateimage sensing element 30/50/60/70 forms a part of the array togetherwith other solid state image sensing elements of the same type, and eachof the shift registers is same as the shift register 33/52/62/72. Thehorizontal register and the amplifier are well known to a person skilledin the art, and no further description is incorporated hereinbelow.

The plastic package 91 includes a lead frame 91 a and a piece oftransparent synthetic resin 91 b. The lead frame 91 a has an island 91 cfor mounting the semiconductor chip 90, conductive leads 91 d projectingfrom both sides of the piece of synthetic resin 91 b and conductivewires 91 e connected between the semiconductor chip 90 and theconductive leads 91 d. In this instance, the semiconductor chip 90 isdirectly covered with the transparent synthetic resin 91 b. However, asilicone resin layer may be inserted between the upper surface of thesemiconductor chip 90 and the piece of transparent synthetic resin 91 b.Even if color filters are provided on the semiconductor chips, the colorfilters form a flat upper surface, and the silicone resin layer isinsertable between the color filters and the piece of transparentsynthetic resin 91 b.

The solid state image sensing device is assembled as follows. First, thesemiconductor chip 90 is mounted on the island 91 c, and the conductivewires 91 e are bonded to pads on the semiconductor chip/island 91 b/91 cand the conductive leads 91 d. The semiconductor chip 90 mounted on thelead frame 91 a is placed in a molding die (not shown), and meltedtransparent synthetic resin is injected into the molding die. Then, thesemiconductor chip 90 is sealed in the piece of transparent syntheticresin 91 b. Finally, the conductive leads 91 d are separated from aframe (not shown), and are bend as shown in FIG. 14.

The protective layers 39 on the solid state image sensing elements30/50/70 create the flat upper surfaces, and the piece of transparentsynthetic resin 91 b does not have any influence on the buried miniaturelens regardless of the refractive index of the transparent syntheticresin. However, the protective layer 39 is curved. If the transparentsynthetic resin 91 b is different in refractive index from thetransparent layer 63 a, the piece of transparent synthetic resin 91 baffects the optical characteristics of the buried miniature lens 63.Nevertheless, the optical influence is a little, because the transparentlayer 63 a has a large radius of curvature. Moreover, the opticalinfluence is predictable, and the manufacturer can take the opticalinfluence into account in the design work for the buried miniature lens63. Thus, the semiconductor chip 90 is packaged in the piece oftransparent synthetic resin without space over the semiconductor chip90, and the solid state image sensing device is provided to the marketat low price.

FIGS. 15 and 16 illustrate another solid state image sensing deviceembodying the present invention. The solid state image sensing device issimilar to that shown in FIGS. 13 and 14. The solid state image sensingdevice shown in FIGS. 15 and 16 is sealed in a different kind of plasticpackage 95, and the piece of transparent synthetic resin 91 b isreplaced with a piece of transparent synthetic resin 95 a partiallycovered with a photo-shield layer 95 b. For this reason, othercomponents are labeled with the same references designatingcorresponding components of the solid state image sensing device shownin FIGS. 13 and 14.

The piece of transparent synthetic resin 91 b has a sealing portion 95 cand a convex portion 95 d. The semiconductor chip 90 mounted on the leadframe 91 a is sealed in the sealing portion 95 c, and the convex portion95 d is formed on the upper surface of the sealing portion 95 c. Thesealing portion 95 c is covered with the photo-shield layer 95 b, andlight is incident onto the convex portion 95 d. In this instance, thephoto-shield layer 95 b is formed of black insulating paint. The convexportion 95 d serves as a fixed focus lens. The sealing portion 95 c andthe convex portion 95 d are molded, and, thereafter, the sealing portion95 c is coated with the black insulating paint. The solid state imagesensing device with the fixed focus lens is appropriate for aneconomical camera.

As will be appreciated from the foregoing description, the solid stateimage sensing element according to the present invention does notproject a lens from the transparent layer, and the lens is notmechanically damaged. Moreover, even if the solid state image sensingelement is contaminated with dust particles, the manufacturer easilyeliminate the dust particles from the solid state image sensing elementby using a blower. In other words, the solid state image sensing elementdoes not require perfectly dust-free ambience, and the production costis drastically reduced. The buried miniature lens occupies wide areaover the photo diode and the shift register, and makes the solid stateimage sensing element sensitive.

The protective layer 39 perfectly prevents the solid state image sensingelement from external force and contaminant, and makes the solid stateimage sensing element durable.

The solid state image sensing device according to the present inventiondoes not require any space between a package and the semiconductor chip,and the simple package reduces the production cost.

Although particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the present invention.

In the above-described embodiment, the buried miniature lens 32 has acircular cross section. However, the cross section may be deformed inaccordance with the plane configuration of the photo diode. For example,if a photo diode has a rectangle and the opening of the photo-shieldlayer is a rectangle nested in the photo diode, the buried miniaturelens may be elongated in the direction of the long edge of the rectangleso as to have an elliptical cross section. Then the elliptical buriedminiature lens effectively focuses incident light onto the photo diode.

The protective layer 39 may be deleted from the solid state imagesensing device. The silicon oxide and the silicon nitride are harderthan the photo resist of the second prior art, and the buried miniaturelens 32 and the thick transparent layer 38 are less damaged.

The focal length modifier may be implemented by more than one silicaglass layer.

Any transparent material is available for the focal length modifier 53 aor the convex lens 63 a in so far as it anisotropically shrinks duringthe solidification. Another transparent materials in the silica glasssystem is an example. Yet another example is low-fusing point glass.

If the solid state image sensing element is incorporated in a full colorimage sensing device, a color filter element is inserted between thetransparent insulating layer 34 c and the buried miniature lens, or isprovided on the buried miniature lens.

A convex lens of glass may be adhered to the sealing portion 95 cwithout the convex portion 95 d.

The invention, the present invention is applicable to the MOS type.

What is claimed is:
 1. A process for fabricating a solid state imagesensing element, comprising the steps of a) preparing a substrate; b)forming a photo-electric converting element in a first area of saidsubstrate; b1) forming a photo-shield layer over said photo-electricconverting element and defining an opening in which at least a part ofsaid photo-electric converting element is exposed; c) covering saidphoto-shield layer with a first transparent layer formed of a firsttransparent material; d) forming a mask layer on said first transparentlayer having an opening over a central sub-area of said first area; e)isotrophically etching said first transparent so as to form a firstrecess, wherein said first recess occupies a second area wider than saidfirst area and is shaped independent of the configuration of saidopening; and f) filing said first recess with a second transparentmaterial larger in refractive index than said first transparent materialso as to form a second transparent layer serving as a lens.
 2. Theprocess as set forth in claim 1, in which said step f) includes thesub-steps of f-1) spreading solution of said second transparent materialover said first transparent layer, f-2) solidifying the layer of saidsolution so as to be shrunk into said second transparent layer.
 3. Theprocess as set forth in claim 1, further comprising the step of g)planarizing the resultant structure of said step f) so as to create aflat upper surface.
 4. The process as set forth in claim 3, furthercomprising the step of h) covering said flat upper surface with aprotective layer formed of a third transparent material harder than saidsecond transparent material.
 5. The process as set forth in claim 1, inwhich said second material is silica glass.
 6. The process as set forthin claim 1, in which an etching rate of said first transparent layer isdecreased from an upper surface toward a lower surface.
 7. The processas set forth in claim 6, in which said etching rate is varied bychanging the concentration of an impurity.
 8. The process as set forthin claim 7, in which said concentration of impurity is changed throughan ion-implantation.
 9. The process as set forth claim 7, in which saidfirst transparent layer is formed through a chemical vapor deposition ofa material gas for said second transparent material, and saidconcentration of impurity is changed by controlling an additive gas forsaid impurity mixed with said material gas.